System For Tracking And Maintaining An On-Grade Horizontal Borehole

ABSTRACT

A tracking receiver system is used to track the progress of a downhole tool along a subsurface path having a desired grade. The tracking receiver system is adapted to detect a reference line originating from a reference line receiver and to determine the position of the downhole tool along a desired subsurface path relative to the reference line. In an alternative embodiment the tracking receiver system may comprise a global positioning satellite system to provide information used to determine the position of the downhole tool relative to the desired subsurface path.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority of U.S. Provisional Patent ApplicationSer. No. 60/734,670 filed Nov. 8, 2005 and U.S. Provisional PatentApplication Ser. No. 60/710,523 filed Aug. 23, 2005, the contents ofwhich are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to installation of undergroundutilities and specifically to a tracking system for the trenchlessinstallation of such utilities via horizontal directional drillingon-grade.

SUMMARY OF THE INVENTION

The present invention is directed to an on-grade horizontal directionaldrilling system. The horizontal directional drilling system comprises adownhole tool, a reference line generator, and a tracking receiversystem. The downhole tool is operatively connected to a downhole end ofa drill string and steerable along a desired subsurface path. Thereference line generator is adapted to establish a reference line havinga grade substantially the same as the grade of the desired subsurfacepath. The tracking receiver system comprises a sensor assembly, areference line receiver, a means for measuring the distance between thereference line receiver and the sensor assembly, and a processor. Thereference line receiver is adapted to detect signals emitted from thedownhole tool. The reference line receiver is adapted to detect thereference line. The processor is adapted to process the signals detectedby the sensor assembly and the measured distance to determine theposition of the boring tool relative to the reference line.

The present invention further includes an on-grade tracking receiversystem. The on-grade tracking receiver system is adapted to track theprogress of a boring tool. The boring tool is steerable along a desiredsubsurface path substantially parallel with a reference line having agrade. The tracking receiver comprises a sensor assembly, a referenceline receiver, a means for measuring the distance between the referenceline receiver, and a processor. The sensor assembly is adapted to detectsignals emitted from the boring tool and the reference line receiver isadapted to detect the reference line. The processor is adapted toprocess the signals detected by the sensor assembly and the measureddistance between the reference line receiver and the sensor assembly todetermine the position of the boring tool relative to the referenceline.

The present invention is further directed to a method for creating anon-grade borehole. The method comprises establishing a reference line ata predetermined grade substantially the same as a grade of a desiredsubsurface path and determining a position of a boring tool with areceiving tracker system wherein the receiving tracker assemblycomprises a sensor assembly. The method further comprises detecting thereference line at the receiving tracker system, measuring a distancebetween the reference line and the sensor assembly, and determining theposition of the boring tool relative to the reference line based on thedetermined position of the boring tool and the distance between thereference line and the sensor assembly.

Further, the present invention includes a method for determining aposition of a boring tool relative to a reference line. The referenceline has a selected grade corresponding with a grade of a desired borepath. The method comprises locating the boring tool with a sensorassembly and measuring a distance between the boring tool and the sensorassembly. The method further includes locating the reference line,measuring a distance between the reference line and the sensor assembly,and determining a distance between the reference line and the boringtool based on the measured distance between the boring tool and thesensor assembly and the measured distance between the reference line andthe sensor assembly.

Further still, the present invention is directed to an on-gradehorizontal directional drilling system comprising a downhole tool, atracking receiver system, an optical survey system and a processor. Thedownhole tool is operatively connected to a downhole end of a drillstring and steerable along a desired subsurface path. The trackingreceiver system comprises a sensor assembly adapted to detect signalsemitted from the downhole tool. The optical survey system is adapted tomeasure the range and elevation of the tracking receiver system relativeto a starting above-ground reference point disposed along the desiredsubsurface path. The processor is adapted to process the signalsdetected by the sensor assembly and the range and elevation of thesensor assembly to determine the position of the boring tool.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic representation of a horizontal directionaldrilling system, utilizing an on-grade tracking system constructed inaccordance with the present invention. FIG. 1 illustrates the use of areference line generator disposed proximate to a drill unit.

FIG. 2A is a diagrammatic representation of a tracking receiver systemof the present invention.

FIG. 2B is an alternative embodiment of the tracking receiver systemshown in FIG. 2A. The tracking receiver system of FIG. 2B comprisesphoto detector array movable along an upper pole of the system.

FIG. 3 is a diagrammatic representation of an alternative trackingreceiver system of the present invention having an alternative sensorassembly supported on an extendable frame.

FIG. 4 is a diagrammatic representation of an alternative trackingreceiver system of the present invention having a wireless device formeasuring the distance between a laser receiver and a sensor assembly.

FIG. 5 is a perspective view of an alternative tracking receiver systemof the present invention. The embodiment of FIG. 5 comprises a pluralityof reference line detectors disposed along the length of an extendableframe.

FIG. 6 is a close-up, partially sectional view of a device for manuallyextending the frame of the tracking receiver system of FIG. 5.

FIG. 7 is a diagrammatic representation of the reference line detectorshown in FIG. 5. The reference line detector of FIG. 7 is showncomprising an array of photo detectors.

FIG. 8A is a diagrammatic representation of the tracking receiver systemof FIG. 5 disposed above a downhole tool.

FIG. 8B is a diagrammatic representation of the tracking receiver systemof FIG. 8A in the fully collapsed position to illustrate the distancesbetween each sensor array and the bottom of the sensor assembly.

FIG. 9 shows a diagrammatic representation of an alternative trackingreceiver system of the present invention.

FIG. 10 is a flow chart illustrating a corrective averaging process toaid in comparing the directional downhole tool pitch to the desiredinstallation grade.

FIG. 11 is a flow chart illustrating the adaptation of the process shownin FIG. 10 for automated control of the on-grade boring process.

FIG. 12 is a diagrammatic side view representation of an alternativeon-grade horizontal directional drilling system adapted to use a totalstation survey instrument.

FIG. 13 is a diagrammatic side view representation of an alternativeon-grade horizontal directional drilling system adapted to use a realtime kinematic GPS survey instrument.

FIG. 14 is an overhead view of an enhancement for the tracking receiversystem of the present invention. The enhancement shown in FIG. 14comprises a laser chalk line adapted to aid the operator in maintainingthe desired lateral alignment of the downhole tool with the desiredsubsurface path.

FIG. 15 is a diagrammatic perspective view of the apparatus of FIG. 14.

FIG. 16 is an overhead view of an alternate system to aid in maintainingthe desired lateral alignment of the bore.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Horizontal directional drilling (HDD) permits installation of utilityservices or other products underground in an essentially “trenchless”manner, eliminating surface disruption along the length of the projectand reducing the likelihood of damaging previously buried products. Thetypical HDD borepath begins from the ground surface as an inclinedsegment that is gradually leveled off as the desired productinstallation depth is neared. This interval is generally referred to asthe “set back distance” at which the downhole tool 10 (FIG. 1) mustenter the ground in order to reach the desired depth without violatingbend radius limits of a drill string 12. One experienced in making HDDinstallations acquires a “feel” for the amount of set back required tolevel the downhole tool 10 off at a given depth. However, on criticalinstallations, this segment and other portions of the borepath are oftenpre-planned utilizing surveying instruments and borepath planningsoftware such as described in U.S. patent application Ser. No.10/404,550 “Automatic Path Generation and Correction System” filed Apr.1, 2003, the contents of which are incorporated herein by reference. Theportion of the borepath where the installed product will lay may have aspecified depth (or at least a minimum-maximum range) all along itslength. Alternately, a horizontal or near horizontal path may bedesirable instead.

HDD has proven to be a useful method for the installation of numeroustypes of underground utilities—such as telephone and electric lines andgas and water pipes. One application where HDD has had only moderatesuccess is in the installation of gravity flow liquid conduits such assewer and storm drainage. Installation of gravity flow piping is tediousand requires care to be taken to ensure that the proper grade ismaintained for effective flow of materials through the sewer ordrainage. This is especially important when very shallow grade (<1%grade) is specified. Accordingly, various methods and systems have beendeveloped to conduct on-grade boring operations. However, there remainsa need for improved control of the downhole tool during on-grade boringoperations.

With reference to FIG. 1, there is shown therein an on-grade horizontaldirectional drilling system 14 constructed in accordance with thepresent invention. The HDD system 14 comprises the downhole tool 10operatively connected to a downhole end 16 of the drill string 12, areference line generator 18, and a tracking receiver system 20. Asdiscussed in more detail below, the reference line generator 18 isadapted to establish a reference line 22 having a grade, or slope,substantially the same as the grade of a desired subsurface path 24.

The tracking receiver system 20 may be used to determine the positionand orientation of the downhole tool using “walkover” techniques. Forwalkover tracking, a beacon 26 is disposed within the downhole tool 10and adapted to transmit a magnetic field. A drill unit 28 advances thedrill string 12 and the downhole tool 10 through the ground. An operator30 on the surface of the ground—utilizing the tracking receiver system20—follows the beacon 26 and periodically determines the depth anddirection of the downhole tool 10 utilizing techniques known in the art.

While the present invention is described herein with reference to asingle pipe drill string and a boring tool having a single beacon, itwill be appreciated that a dual-pipe drill system and tooling asdisclosed in U.S. Pat. No. 6,827,158 may be utilized with the invention.Additionally, an alternative version of the two pipe system as disclosedin previously referenced U.S. patent application Ser. No. 10/724,572having a trailing second beacon may be used in accordance with thepresent invention.

Basic walkover style position and orientation sensing systems aredescribed in U.S. Pat. No. 5,264,795 issued to Rider, U.S. Pat. No.5,850,624 issued to Gard, et. al., and U.S. Pat. No. 5,880,680 issued toWisehart, et. al., the contents of which are incorporated herein byreference. Sensors for determining the orientation of the downhole tool10 are described in the latter two patents as well as in U.S. Pat. Nos.5,133,417 and 5,174,033 issued to Rider and U.S. Pat. No. 5,703,484issued to Bieberdorf, et. al., the contents of which are alsoincorporated herein by reference.

In FIG. 1, dashed lines 32 directed diagonally upward toward the rightindicate the surface of the ground. The HDD unit 28 is set up at apre-planned position such that the downhole tool 10 reaches a desireddepth, grade and left-right alignment at the start of the on-gradeportion of the bore. Generally, this location will coincide with astarting pit 34. The downhole tool 10 is shown entering the ground atthe forward end of the starting pit 34—illustrated by a cut-away of thesoil overburden. The downhole tool 10 may be supported by a crane orbackhoe (not shown) as it passes through the starting pit 34, so minoradjustments in depth and alignment are possible prior to groundre-entry. Proper positioning at this point 36 ₀ and at selected otherlocations 36 _(i) along the on-grade segment 24 of the borepath isverified by the tracking receiver system 20. As the bore progresses, thetracking receiver system 20 is moved directly above of the downhole tool10 using techniques known in the industry.

A preferred practice with the method of the present invention is topre-mark the desired subsurface path 24 on the ground 32 with a tautstring, marker flags 38, paint marks or some other marking method. Bydoing so, the line (right/left deviation) of the bore may also bemonitored closely to assure that it does not wander out of the allottedright/left horizontal window for the bore. Flags 38 may be disposed onthe ground 32 at points between the operator 30 and the starting pit 34above where the downhole tool 10 has traveled.

Certain tracking receiver systems allow depth readings to be obtainedwithout being directly above the beacon 10. One example is disclosed inU.S. Patent Application No. 60/680,780 “Dipole Locator UtilizingMultiple Measurement Points” filed May 13, 2005, incorporated herein byits reference. When incorporated into the present invention, thistracking receiver system may be positioned over the desired subsurfaceborepath 24 and step-wise advanced to stay within range of the beacon26.

The tracking receiver system 20 of the present invention typicallyincludes a tracker 40 having a conventional sensor assembly, a displayand controls (not shown) for use by the operator 30. The tracker 40 mayalso have a radio link 42 for single or bi-directional communication ofdata 44 to and from a display unit (not shown) mounted at the HDD unit28 to provide information related to the bore to an operator stationedat the HDD unit. Alternatively, communication may be with a system 46 atthe drill unit 28 that automatically operates and coordinates thevarious functions comprising the drilling operation. Such an automatedcontrol system is disclosed in commonly assigned U.S. Patent ApplicationPublication No. 2004/0028476 “System and Method for AutomaticallyDrilling and Backreaming a Horizontal Bore Underground”, the contents ofwhich are incorporated herein by reference. As used herein, automaticoperation is intended to refer to a drilling interval or sequence ofdrilling operations that can be accomplished without operatorintervention and within certain predetermined tolerances.

Continuing with FIG. 1, the tracking receiver system 20 may comprise anextendable frame 100 adapted to support a sensor assembly 92 having aplurality of horizontally separated antenna arrays 94 for detectingsignals from the beacon 26 and a plurality of reference line detectors96 vertically spaced along the frame. Alternatively, a sensor assemblywith a single antenna array set may be utilized with the currentinvention. The antenna arrays 94 may each comprise a set of tri-axialantennas adapted to detect a magnetic field signal emitted from thebeacon 26 and are supported by a lower pole 102 having a foot 104. Thereference line detectors 96 may comprise photo detector arrays that aresupported on or within the upper pole 98 or extension pole or poles ofthe frame 100. A manually actuated device 106 may be supported on theframe 100 and adapted to move the upper pole 98 relative to the lowerpole 102 to raise and lower the detectors 96. The tracking receiversystem of FIG. 1 is discussed in more detail with reference to FIGS.5-8.

Referring still to FIG. 1, once the start point 36 ₀ of the on-gradeportion of the bore is reached, the reference line generator 18 ispositioned in line with the intended on-grade borepath 24. The referenceline generator 18 is adapted to establish the reference line 22. Thereference line 22 has a grade substantially the same as the grade of thedesired subsurface path 24. In accordance with the present invention,the reference line generator 18 may comprise a laser transmitter adaptedto generate a rotating beam laser level projected as a plane, generallypositioned a few feet above the ground surface 32. A pedestal 52 ortripod-mounted reference line generator 18 readily allows this plane tobe inclined off horizontal at a particular slope (grade) in one selecteddirection. A commercially available reference line generator is theTopcon RL-H3C series and its associated laser receiver. Whatever gradethe bore is to proceed at, whether positive or negative, the referenceline generator 18 will be set up such that a reference line 22 projectsthe target grade (rise 54/run 56) specified for installation of theproduct pipe along its desired heading. When a rotating laser plane isused as the reference line generator 18, the vector of reference line 22lies within a projected plane whose axis is parallel to the bore path ata fixed distance above the on-grade segment of the planned borepath. Theaxis of the plane perpendicular to the desired bore path should be sethorizontal or at a 0% grade. In the case of a rotating laser plane, theprojected plane will be used as a reference elevation standard at eachof the various tracking locations 36 _(i) on the ground surface 32 alongthe intended on-grade borepath 24. These locations 36 _(i) may be spacedat regular, reasonably close intervals (e.g., 2 to 5 feet apart).

Turning now to FIG. 2A, a tracking receiver system 20 of the presentinvention is shown. The tracking receiver system 20A comprises areference line receiver 50 and the tracker 40. The tracker 40 comprisesa sensor assembly 58 adapted to detect signals from the downhole tool 10(FIG. 1). The sensor assembly 58 comprises at least one antenna adaptedto detect the signal emitted from the beacon 26 (FIG. 1). In theembodiment of FIG. 2A the sensor assembly 58 comprises a tri-axialantenna array.

The tracker 40 is supported on an extendable frame 62. The extendableframe 62 comprises a base 60, a first member 64 connected to the base60, and a second member 66 operatively connected to the first member 64and adapted to support the reference line receiver 50. The referenceline receiver 50 shown in FIG. 2A comprises a laser receiver adapted toreceive the reference line 22. The second member 66 is also adapted tosupport a means for measuring a distance between the reference linereceiver 50 and the sensor assembly 58. The means shown in FIG. 2Acomprises a string potentiometer 68 supported on the frame 62 to measurea change in distance between the reference line receiver 50 and the base60 when the second member 66 is moved up or down relative to the firstmember 64. Additionally, reference line receiver 50 mounted on secondmember 66 with an adjustable clamp 65 and may be moved along secondmember 66. The string potentiometer 68 is connected to the tracker 40via cable 70 to transmit data indicative of the distance between thereference line receiver 50 and the base 60. Alternatively, thecommunication between the string potentiometer 68 and the tracker 40 maybe accomplished by wireless means. One skilled in the art willappreciate that the alternative distance measuring devices such as anoptical device, ultrasonic device, or other means may be mounted onframe 62 to measure the distance between the reference line receiver andthe base 60. It will also be apparent that the string potentiometer 68could be co-mounted with the reference line receiver 50 and the point ofattachment for the end of the measurement cable could be located on thebase 60 of the device. Similarly, optical, ultrasonic, or othermeasurement devices could be adapted to mount either on the bracketholding the reference line receiver 50 or alternatively, mount on thebase 60.

With reference now to FIG. 2B, an alternative embodiment of the trackingreceiver system 20 shown in FIG. 2A is illustrated therein. The trackingreceiver system 20B of FIG. 2B comprises a tracker 40 supported onextendable frame 62. The extendable frame 62 comprises the base 60, thefirst member 64 connected to the base and the second member 66operatively connected to the first member. The second member maycomprise an upper pole adapted to move up or down relative to the firstmember 64. The clamp 65 is supported on the extendable flame to securethe second member 66 relative to the first member 64 to fix the heightof reference line receiver 50 a above the ground.

The reference line detector 50 a shown in FIG. 2B comprises a photodetector array comprising a plurality of linearly arranged sensorssensitive to the wavelength of reference line 22 comprising a laser. Thephoto detector array 50 a is supported to the upper pole 66 a with asliding clamp 67 having a locating tab 69 that limits the clamp todetent positions 71 along the upper pole 66 a. The photo detector array50 a is capable of determining which sensor is impinged by the laser anddetermining the vertical distance between the sensor and the locatingdetent. Sensors and contacts (not shown) may be used to identify eachdetent and it corresponding position relative to the tracker 40. Thedistance between the impinged sensor and the contacted detent and thedistance between the contacted detent and the tracker 40 are added todetermine the height of the reference line 22 of the base of thetracker. It will be appreciated that the extendable frame 62 may alsocomprise a detent and sensor system to determine the relative positionof the upper pole 66 a and the lower pole 64. This measurement may thenbe included in the total distance measurement to provide a greatermeasurement range.

Referring now to FIG. 3, an alternative tracking receiver system 20C ofthe present invention is comprised of a bracket 72, or similar clampingarrangement, which couples an upper pole 74 and a foot 76 equipped lowerpole 78 to an alternative tracker 80. For ease of use, the tracker 80may mount rigidly to the bracket 72 and not move relative thereto.

Turning now to FIG. 4, there is shown therein an alternative embodimentof the tracking receiver system of the present invention. The embodimentof FIG. 4 comprises a sensor assembly 84, a reference line receiver 50supported above the sensor assembly by an extendable frame 48, and ameans for measuring 88 the distance between the reference line receiverand the sensor assembly. The measuring means 88 of the embodiment shownin FIG. 4 comprises a dipole magnetic field transmitter 91 supported onthe upper pole 74 and the sensor assembly 84 supported by a tracker 80.In operation, the dipole magnetic field transmitter 91 transmits amagnetic field 90 that may be detected by antennas of the sensorassembly 84. The magnetic field 90 transmitted from the transmitter 91may be transmitted at a different frequency from the field transmittedfrom beacon 26 disposed within the boring tool 10. The detected signalsare processed by the tracker 80 to in a conventional manner (mostcommonly from a calibrated relation with received magnetic fieldstrength) to determine the distance between the reference line receiver50, supported by the upper pole 74 and co-mounted with the transmitter91, and the sensor assembly 84. This distance is stored in the tracker's80 memory and later used to determine the distance from the referenceline receiver 50 and the downhole tool 10 (FIG. 1).

With reference now to FIG. 5, there is shown a more detailedrepresentation of the tracking receiver system illustrated in FIG. 1.The tracking receiver system 20 preferably comprises a sensor assembly92 having a plurality of horizontally separated antenna arrays 94 fordetecting signals from the beacon 26 (FIG. 1) and a plurality ofreference line detectors 96 vertically spaced along the extendable frame100. Alternatively, a sensor assembly with a single antenna array setmay be utilized with the current invention. The antenna arrays 94 mayeach comprise a set of tri-axial antennas adapted to detect a magneticfield signal emitted from the beacon 26 (FIG. 1) and are supported by alower pole 102 having a foot 104. The reference line detectors 96 maycomprise photo detector arrays that are supported on or within the upperpole 98 or extension pole or poles 136 of the frame 100. A manuallyactuated device 106 may be supported on the frame 100 and adapted tomove the upper pole 98 relative to the lower pole 102 to raise and lowerthe detectors 96.

Referring now to FIGS. 5 and 6, the manually actuated device 106preferably comprises a rack 110 and pinion drive (not shown) driven by acrank handle 108. It will be appreciated that other means of moving theupper pole 98 with respect to lower pole 102 are possible. These mayinclude manual sliding arrangements, screw drives, friction drives, orother means. In the present embodiment, the rack 110 is supported on theupper pole 98 and the crank handle 108 and driving pinion are supportedon the lower pole 102. Actuating the crank handle 108 to drive pinionwill cause the upper pole 98 to move up or down relative to the lowerpole 102 depending on the direction in which the handle is rotated. Abrake knob 112 may be used to lock the poles 98 and 102 relative to eachother. An embedded magnetic strip 114 supported by the lower pole 102and an embedded magnetic encoder sensor 116 may be supported by theupper pole 98 may be used to measure the position of the upper polerelative to the lower pole. Alternatively, a string potentiometer,various optical devices, rotary encoders on the handle 108, ultrasonicdevices, or other means may be used to measure this relativedisplacement. This information may then be used, in manner yet to bedescribed to determine the distance between the reference line detector96 being impinged by the reference line 22 and the horizontal plane ofthe sensor assembly 92.

Turning now to FIG. 7, one of the reference line detectors 96 of FIG. 5is shown in detail. The reference line detector 96 is comprised of aplurality of photodetecting sensors 118-132 arranged in a linear arrayhaving a height H_(detector). Each sensor 118-132 has an individualheight H_(sensor). In the embodiment of FIG. 7 H _(sensor) includes theheight of the sensor 118-132 plus any vertical distance between adjacentsensors. Each sensor 118-132 produces an electrical current whenimpinged upon by a light of the appropriate wavelength, such as the ofthe reference line laser beam. Within a sensor there is nodifferentiation of the exact point of contact by the light.Transimpedance amplifiers associated with each sensor convert thecurrent produced by the photodiode into a voltage. This voltage is fedinto a comparator (not shown) where it is compared to a thresholdvoltage. The resulting discrete signal indicating whether the sensor hasbeen illuminated is latched for later use. In FIG. 7, the sweeping spotof the reference line 22 is indicated by circular region 134. In thisexample sensors 4, 5, and 6 are illuminated by the reference line 22.Accordingly, the point of incidence of the reference line 22 isindicated by the center of the illuminated sensors. In this case, thecenter of the sensors illuminated would be the center of sensor 5.

One skilled in the art will appreciate other configurations of photosensors such as a staggered pattern could also be used and could improveresolution. In the preferred embodiment, 8 photodiodes such asPDV-C173SM made by Advanced Photonix are used in each array, but thearray could consist of any odd or even number of photo sensitiveelements and could be constructed with different types of sensors suchas photo transistors. The photo diodes used in the current embodimentare spaced such that H_(sensor) is 4.25 mm. This arrangement provides ameasurement resolution of ½*H_(sensor)=2.125 mm. The photodiodes used inthe detectors 96 preferably have some type of optical filtering to limitthe wavelengths of light impinging on them and reduce the amount ofunwanted received ambient light. The combination of the photodiodes usedand their optical filtering is intended to allow light wavelengths from600 to 950 nm to pass. It should be obvious that the filters could bedesigned to favor different wavelengths to accommodate different lightsources.

With reference now to FIG. 8A and as previously discussed reference linedetectors 96 of tracking receiver system 20 may comprise multiplediscrete microcontroller monitored laser detectors used in conjunctionwith the previously disclosed sliding frame arrangement to determine thevertical displacement HL of the reference line 22 above the trackingdevice. One skilled in the art will appreciate the detector may bemonitored or controlled by digital or analog logic. The individualdetectors 96 are supervised by a main microcontroller (not shown) whichcollects data from each detector and the encoder 116 (FIG. 6) andprovides the operator interface. In the embodiment of FIG. 8A, twodiscrete detectors 96 are located in the upper pole 98 and one detectoris located near the top of each extension pole 136. It will beappreciated that multiple detectors 96 may be placed on the upper pole98 and/or on each extension pole 136. Use of extension poles 136effectively adds length to the base survey pole. Each extension pole 136has a mechanical connector and an electric plug (not shown) near thebottom for connection to the pole immediately below it and a receptaclefor accepting the plug from the pole immediately above it. An additionalcap piece 138 is used to terminate electrical connections andmechanically seal the topmost extension pole 136 or the upper base poleif no extension poles are in use. The cap 138 may also contain aconstant current source used to generate reference voltages thatidentify different on-grade indications. A plug type electricalconnector (not shown) may contain power, ground, an analog referenceline with a constant current flowing through it, and one wire for theHigh, Low, and On Grade (analog) signals.

Referring now to both FIGS. 8A and 8B, vertical displacement from thesensor assembly 58 to the center of each detector 96 is known and may bedenoted H_(DX) where _(X) is the detector number. The magnetic stripencoder 116 (FIG. 6) is used to measure the relative displacement of thetwo poles H_(R). While this is used in the present embodiment of theinvention, it should be obvious that different types of encoders such asrotary, optical, or others could also be used. It is also obvious thatvarious other types of measurement systems could be used to determinethe distance H_(R) such as ultrasonic, laser, or string potentiometer.H_(DX) and H_(R) can be added together to find H_(L) when the referenceline 22 is incident on the center of one of the detectors 96. Eachdetector 96 determines whether the laser beam is High, Low, or On Graderelative to the center of the detector and relays this information to amain microcontroller (not shown). The main microcontroller combines thisdata with the encoder data to determine the vertical displacement H_(L)of the reference line above the sensor assembly. This information may berelayed by radio to the tracker for further use.

In operation and as shown in FIG. 8B, the operator 30 lowers the upperpole 98 to its minimum height and turns on the electronics. Thisestablishes the zero reference for H_(R) (FIG. 8A). In the currentembodiment, an incremental encoder is used which would require a zeroreference point. An absolute encoder may be used, however, to eliminatethe need for this reference and simplify the setup procedure. Theoperator then adjusts the height of the upper pole 98 so that theapproximate center of one of the detectors 96 is impinged by thereference line 22. This may be done by observing a set of threeindicator LEDs (not shown). A yellow LED may indicate that the referenceline 22 is high relative to the center of the detector; a red LED mayindicate that the reference line is low relative to the center of thedetector, and a green LED may indicate that the reference line isimpinging the center of the detector 96. The LEDs will only turn on whenone of the detectors 96 is receiving light from the reference line 22,thus there will be large ranges where the operator will see no indicatorLEDs and will need to adjust the height of the upper pole 98 until oneof the LEDs turns on. When the green LED is on, the operator can press adata request button (not shown) which, in the current embodiment of theinvention, causes the main microcontroller to display H_(L) on an LCD140 or other display device (FIG. 5). It should be obvious that variousother means of operator interface could be used in lieu of a pushbuttonor LEDs. Though the preferred embodiment uses colored LEDs to indicatethe position of the detector 96 relative to the laser plane, other meansof operator interface could be employed including incandescent lamps,graphical displays, and audible tones. The operator interface could alsobe located in or integrated into the tracker or other device andoperably connected to the survey pole electronics by radio or othermeans.

A processing algorithm may be used to determine whether the referenceline 22 is impinging any of the detectors 96 and if so, whether thereference line is High, Low, or On-Grade with respect to the center ofthe detector. In order have a valid On-Grade indication; the entirereference line 22 spot should impinge upon the sensor array 96. Sensors1 and 8, the top and bottom sensors of the array 96 respectively, shouldnot be impinged by the reference line 22. If one or both of theseoutside sensors 96 were impinged upon by the reference line it would bedifficult to determine the location of the center of the reference line22 because part of the beam spot could be outside the detector. Themicrocontroller (not shown) first checks to see if sensor 1 was hit. Ifso, the incident laser beam is High. Similarly, if sensor 8 was hit, thebeam is Low. The processing algorithm can then ignore sensors 1 and 8 inits remaining calculations.

A detection sum is then computed based on the detection sensors hit bythe reference line 22. Sensor 2 is given a weight of 2, sensor 3 isgiven a weight of 4, sensor 4 is given a weight of 6, and the patterncontinues until sensor 7 is given a weight of 12. The detection sum iscomputed by adding the weights of all of the sensors 96 that were hit.The detection sum is then divided by the number of sensors 96 hit toobtain the detection quotient used determine the location of the centerof the incident reference line 22. The detection quotient indicates thelocation of the center of the incident reference line 22 in terms of theweights assigned to each sensor with a resolution of ½ H_(sensor). Ifthe detection quotient matches any of the weighted sensor values, itindicates that the reference line 22 is centered on that sensor. Adetection quotient between the weights of two sensors 96 indicated areference line 22 centered between the two sensors.

An “On-Grade” indication is given if the detection quotient is 6, 7, or8 indicating that the center of the reference line 22 is located at orbelow the center of sensor 4 and at or above the center of sensor 5.Using this method, the detector 96 is capable of determining the centersof reference line 22 beam spots of varying diameters. An On-Gradeindication is possible for reference line 22 widths ranging from lessthan the height of a single sensor to the combined height of 6 sensors.For detection quotients less than 6, the reference line 22 is High withrespect to the center of the detector, and a High indication is given.For detection quotients greater than 8, the reference line 22 is Low.The microcontroller then updates discrete outputs to indicate theposition of the reference line 22 using positive logic for beampresence. The microcontroller waits an appropriate time period beforescanning the latches. In the embodiment disclosed herein, themicrocontroller waits 100 milliseconds before scanning the latches. Thisensures that the reference line 22 sweeps past the detector 96 again. Inthis embodiment the reference line 22 comprises a rotating laser planespinning at approximately 600 RPM. Laser planes spinning faster than 600RPM will work well with the current program. For example, a laser planespinning at 1800 RPM would sweep past the detector 3 times before themicrocontroller read the latched data. Since the data is latched, datafrom all 3 sweeps would be logically ANDed together and processed by themicrocontroller as usual. The program could be modified to handle laserplanes rotating at lower speeds by increasing the delay time beforereading the inputs and resetting the latches.

Referring still to FIGS. 8A and 8B, the main microcontroller collectsdata from the individual detector microcontrollers and the encoder 116in order to determine H_(L). During initialization, immediately afterpower-up, the number of encoder counts is zeroed as the survey pole isfully collapsed (FIG. 8B) before it is turned on. Other methods ofzeroing the number of encoder counts could be employed and would bewithin the scope of this invention. For example, on power up, theencoder counts could be zeroed. Then any time the encoder count sum wentnegative, it would be reset to zero. In this way, the pole could bepowered up at any height and then collapsed to its minimum height tozero the encoder count. Also during initialization, the encoder inputsare set up to generate interrupt requests to ensure that encoder pulsesare given the highest priority. The microcontroller keeps track of theencoder counts using interrupt subroutines that effectively run in thebackground. The microcontroller begins the main program by polling theHigh and Low discrete signals and the On-Grade analog signal andupdating the indicator LEDs accordingly. This process continues untilthe operator presses the data request button. At that time, themicrocontroller checks that the On-Grade indication is present. If notpresent, an error message is displayed after which the user must pressthe data request button again to return to normal operation. If theOn-Grade indication is present, the microcontroller then looks at theanalog input voltage to determine which detector is sending the On-Gradesignal. Then the distance HL is calculated by multiplying the number ofencoder counts by a conversion factor to obtain a distance in units ofthe operator's choice, then adding H_(DX) and finally adding a globaloffset that can be used to zero out the system. In the currentembodiment, H_(L) is then displayed on the LCD until the operator againpresses the data request button. Program flow then returns to pollinginputs and updating outputs until the data request button is pressedagain.

Continuing with the embodiment of FIG. 8A, after the survey poleelectronics relay the distance H_(L) to the tracker or rig by RF orother means, the data could be presented to the operator in variousways. The vertical distance E_(L) from the downhole tool 10 to thereference line 22 could simply be displayed for the operator 30 (FIG. 1)who would then make bore path corrections based on this number.Alternatively, a zero offset scheme could be implemented. In such ascheme the downhole tool 10 would be in the ground in the properorientation at or near the starting pit 34, the reference line 22 wouldbe set up at the appropriate grade, and the operator 30 would bedirectly above the downhole tool 10 with the tracking receiver system20. The tracking receiver system operator 30 would then adjust theheight of the upper pole 98 to locate the reference line 22 and thenpress a button or other interface on the tracker 80 or on the poleitself to trigger measurement of the target vertical distance E_(L) ofthe reference line 22 above the downhole tool 10. The initial E_(L)distance would be used as the target for the remainder of the bore.Grade drilling would then proceed until another depth measurement wastaken. In this embodiment, a positive value would indicate that thedownhole tool 10 was shallower than the desired bore path whereas anegative value would indicate that the downhole tool 10 was deeper thanthe desired bore path. The positive and negative conventions could alsobe reversed to make them more intuitive to the operator 30 of the HDDunit 28 (FIG. 1).

To determine the distance E_(L) between the reference line 22 and thedownhole tool 10, the downhole tool is located with the trackingreceiver system 20 using known techniques. The depth of the downholetool 10 from the tracker sensor assembly 58 is obtained using the sensorassembly and is represented by D_(M). Either simultaneous with orsequentially with the determination of D_(M), the distance HL betweenthe reference line 22 and the sensor assembly 58 is determined asdiscussed above. Once H_(L) has been determined by the electronicssupported by the frame 100, this measurement is sent to the tracker 80.Transmission of information between the frame electronics and thetracker 80 may be via wired connection or a wireless link. Once H_(L)and D_(M) have been obtained by the tracker 80, they may be summed todetermine the resultant elevation E_(L).

In the present embodiment, the value H_(L) is the sum of a number ofquantities related to the construction of the tracking receiver system20. This value, as shown in FIG. 8A, includes H_(DX), the distance fromthe centerline of the detector 96 sensing the reference line to the baseof the tracker 80 when the upper pole 98 is in the lowest positionrelative to the lower pole 102. The values of L_(L) (the distancebetween detectors on the upper pole 98) and L_(E) (the height of anextension pole 136) are both equal to one meter. Other convenient valuessuch as one yard could also be used. It is not necessary that the valuesof L_(L) and L_(E) be equal, but is preferred. H_(L) also includes thelength of relative movement between the lower pole 102 and the upperpole 98 represented as H_(R). This distance may be measured using thepreviously discussed embedded magnetic strip 114 and encoder 116.

In an alternative embodiment, the detector microcontrollers (not shown)may be programmed to locate the center of the incident laser beam andrelay this data to the main microcontroller (not shown) via a serialcommunication or other means. In such an arrangement the distancebetween the center of the detector and the center of the incident laserbeam could be referred to as d_(c) (FIG. 7) and would, in the abovecase, be equivalent to ½ of the sensor height H_(sensor). In thisembodiment H_(L) may include H_(DX), H_(R), and the correction valued_(c) that accounts for the distance between the point at which thereference line 22 actually impinges upon the detector 96 and thecenterline of the detector.

With reference now to FIGS. 1 and 8A, use of the tracking receiversystem 20 to monitor the depth of the downhole tool 10 will bediscussed. Using the tracking receiver system 20 of the presentinvention a target distance or elevation, E_(L0), between the referenceline 22 and the downhole tool 10 is established at or near the edge ofthe starting pit 34, or at the beginning of the on-grade section of thebore 24 if no starting pit is utilized. The target distance, E_(L0), isacquired by locating the position of the downhole tool 10 in thehorizontal plane using the sensor assembly 92. The sensor assembly 92 ispreferably located directly above the beacon 26 in the downhole tool 10,but when using the sensor assembly 92 of FIGS. 1 and 8A comprisingmultiple antenna arrays 94 the tracking receiver system may be laterallyoffset from the borepath and still obtain a valid reading. The foot 104of the lower pole 102 is then placed on the ground 32 and the upper pole98 is raised or lowered until the center of the detector 96 is impingedby the laser beam 22 in a manner previously described. At this point areading is taken with the sensor assembly 92 to measure the depth of thedownhole tool 10 below the sensor assembly. Either sequentially orsimultaneously with the measurement of the depth of the downhole tool10, the height H_(L) of the reference line 22 above the sensor assembly92 is determined as previously described. Since the reference line 22 issubstantially parallel to the intended on-grade path 24 of the downholetool 10, the object is to hold subsequently measured distances E_(Li)substantially equal to the reference distance E_(L0) throughout theon-grade bore.

At each of several tracking locations 36 or stations along the desiredpath 24 of the bore, the foot 104 of the tracking receiver system 20 isplaced on the ground surface and the extendable frame 100 is adjusteduntil the center of the detector 96 is impinged by the reference line 22as determined in a manner previously described. The data request button(not shown) of the tracking receiving system 20 is pushed to obtain areading of the depth, D_(M), of the downhole tool 10 below the sensorassembly 92 and to obtain the height H_(L) of the reference line 22above the sensor assembly. Having obtained these two values, the totaldistance E_(Li) of the reference line 22 above the downhole tool 10 maybe calculated. The value of E_(Li) may be compared to the referencereading E_(L0) to determine the need, if any, of a steering correction.However, provisions are present in some tracking receiver systems orremote displays 46 allowing depth readings to be sequentially recordedinto memory. These values may be recalled from memory to aid in thecomparison. Additionally, a processor may compare the latest distancereading with the initial reading and determine whether the bore isprogressing within the specified grade design tolerance. If out oftolerance, audible warnings could be generated or a stop command (plusadditional commands) sent to the automated control system 46 that mayhave operational control of the drill unit 28. It will be appreciatedthat any of the embodiments of this disclosure may be used in a similarmanner to guide the on-grade bore. Each would employ the unique featuresof that embodiment to provide the operator with a single value tocompare to an initial target value to determine whether or not the boreis progressing on-grade.

A bubble level or other level measurement device (not shown) may beincorporated into the tracking receiver systems disclosed herein to aidthe operator 30 in holding the tracking receiver system vertical, orplumb, as the reading is being taken. The bubble level or plumbindicating device may be incorporated as a virtual “bubble level” intothe display 140 on the tracker 80, or may comprise a physical devicemounted on tracking receiver system. When the bore is performed in thisfashion, each elevation reading should be substantially the same as thetarget reading E_(L0) measured at the start of the on-grade section ofthe bore 24. With this information, the drilling unit operator ortracker operator 30 may quickly discover any deviation from the desiredgrade and determine what sort of directional correction is needed toreturn the bore to desired grade.

The method and apparatus of the present invention eliminates the need toshoot a topographic map of the bore operation area before beginning theoperation. Topographic mapping of an area using prior art trackingdevices is inadequate because local anomalies as small as rodent moundsor wheel ruts in the surface of the ground can cause erroneoustopography-based depth readings. The method and apparatus of the presentinvention resolves this issue because depth measurements are taken eachtime with reference to the stable laser beam 22.

While the implementation of this method discussed above includes the useof a laser plane established on-grade, it is also conceived that othermeans could be used to accomplish the same task. For example, an opticaltransit could be established near the start of the bore in place of thelaser line generator 18. The optical transit would have to be adjustedsuch that the line of site in the direction parallel to the desired borepath 24 was at the desired grade of the bore. This method would beparticularly suited for use with the devices of FIGS. 2, 3, 4, and 14. Asimple optical target could then be mounted on the extendable frame 62(as shown in FIG. 2, or its equivalent in the alternate figures) inplace of the reference line receiver 50. One worker could sight throughthe transit and provide indication to the operator 30 when the frame ofthe tracking receiver system is properly adjusted instead of relying onaudible or visual signals from the laser receiver for this information.Once the optical target was at the correct level, a depth reading of theboring tool 10 could be taken along with the elevation of the opticaltarget with respect to the tracker and that combined reading compared tothe target depth established at the start of the bore.

In cases where the line of sight between the reference line generator 18and the reference line receiver 50 is obstructed, the reference linegenerator may be laterally offset from the directional heading of thedesired borepath 24 or laterally offset along the desired borepathbeyond the starting pit 34. The inclination of the rotating laser beam22 plane may be set equal to the desired grade of the bore in thedirection parallel to the desired bore path, and set horizontal (0%grade) in the direction perpendicular to the direction of the desiredbore path in these instances. Alternately, a tracker 80 (FIG. 3) thatallows depth readings to be obtained without having the trackerpositioned directly above the downhole tool 10 may be utilized in thetracking receiver system. This type of tracker 80 can be moved laterallyoff the intended alignment of the borepath to the point where theobstruction no longer shields the laser beam 22.

Comparing downhole tool 10 position and heading information obtained bythe tracking receiver systems of the present invention to the desiredpath 24 determines whether a steering correction is necessary and, ifso, its proper direction. If a steering correction is not needed, thedownhole tool 10 is advanced with rotation. Typical directional downholetools 10 are generally advanced with rotation to counteract itsdirectional features of the boring tool to continue on its presentheading.

When using the present method, if the indicated depth of the beacondeviates from the target depth, the operator of the rig should not basehis steering correction solely on getting back to the target depth forthe bore. When boring at a negative grade (downhill), correcting a“deep” reading too rapidly can result in a dip in the bore path.Conversely, when boring at a positive grade (uphill), correcting ashallow reading too quickly may also result in a dip in the bore path.Instead, in these circumstances, when the deviated reading isencountered, the pitch at which the boring tool is advanced should beslightly altered in the required direction. In doing so, the depth willgradually be brought back to the target value without creating a lowspot, or dip, in the bore path. The key is that when the desired borepath lies on a negative grade, the pitch at which the head is advancedshould not be allowed to go positive. When the desired bore path lies ona positive grade, the pitch at which the head is advanced should not beallowed to go negative.

Because it may be difficult for the operator 30 to monitor and mentallyaverage displayed pitch readings “on the fly” and visually detect anysubtle change when the beacon 26 and its pitch sensor are being rotatedas fast as 150 to 250 revolutions/minute in the straight drilling mode.This factor, coupled with vibratory effects from a rotating downholetool 10 can contribute toward sporadic false pitch readings.Commercially available beacons such as the Subsite® 86BG grade beaconare able to discern slow rotation to orient the boring tool for asteering correction when they are being rotated at drilling speeds.Thus, when high-speed rotation is detected the beacon's processor (notshown) is programmed to drop other data streams and transmit essentiallyonly averaged pitch sensor readings. U.S. Pat. No. 5,703,484 issued toBieberdorf, et. al. the contents of which are incorporated by referenceherein, discloses a beacon processor utilized to “average” samples ofpitch data before transmission. Averaged pitch values can then be sentat the throughput capacity of the amplitude or frequency modulationtechniques being applied to the carrier frequency transmitted by thebeacon.

Turning now to FIG. 9, an alternative embodiment of the trackingreceiver system of the present invention is shown therein. The trackingreceiver system 20D comprises a frame 200 having an upper pole 202 and alower pole 204. The upper pole 202 supports a sensor assembly 206. Thesensor assembly 206 may comprise a lower housing 208 and an upperhousing 210, but other form factors for the sensor assembly may be used.The lower housing 208 may be adapted to support an antenna assemblyadapted to detect signals transmitted from beacon 26 (FIG. 1). The upperhousing 210 may comprise a handle 212 and a display (not shown).Further, a wireless communication system 214 may be supported by theupper housing 210. The sensor assembly 206 is supported on the frame 200using a bracket 216 that is adapted to allow movement of the upper pole202 and the sensor assembly 206 relative to the lower pole 204. Thebracket 216 may comprise a clamp device (not shown) to lock the positionof the upper pole 202 and sensor assembly 206 relative to the lower pole204 while readings are taken. A reference line receiver 50 may also besupported on the upper pole 202 of the frame 200.

In operation, the tracking receiver system 20D is used to locate thedownhole tool 10 along the desired subsurface path. Once the downholetool 10 is located, the upper pole 202 and sensor assembly 206 areraised or lowered until the reference line receiver 50 is impinged bythe reference line 22 (FIG. 1). It will be appreciated that the distancebetween reference line receiver 50 and the antenna assembly in housing208 will remain substantially constant as the upper pole 202 and sensorassembly 206 are adjusted relative to lower pole 204. An LED or othervisual indicator may be used to indicate to the operator that thereference line receiver is being impinged by the reference line. Oncethe reference line receiver is being impinged by the reference line, adepth measurement may be taken by the sensor assembly 206 and displayedon the display (not shown) of the upper housing 210. In using thisdevice, a target depth is obtained at or near the start of the on-gradeportion of the bore. The depth measured by the sensor assembly 206 atany point along the on-grade section of the bore is then compared to thetarget depth to determine of the downhole tool 10 is proceeding alongthe desired subsurface path at the desired grade. It will be appreciatedthat to allow this system to handle larger changes in elevation alongthe bore, extension poles (not shown) of known length may be added ontop of upper pole 202 and reference line receiver 50 moved to a knownposition near the top of each extension pole. When an extension pole isadded to the system, the target depth must be adjusted by subtractingthe length of each extension pole from the original target depth. Whenusing the device of FIG. 9, the only required measurement at eachlocation point is the depth of the downhole tool relative to the sensorassembly 206. It is unnecessary to obtain a separate reading of distancefrom the reference line receiver 50 to the sensor assembly 206 as thisdistance will remain constant unless altered by the inclusion ofextension poles as previously discussed.

With reference now to FIG. 10, shown therein is a basic flow chart forimplementing additional averaging of the beacon's pitch readings P_(i).This subroutine would be called upon at Start 900 when the downhole tool10 enters the on-grade segment (FIG. 1) of the bore 24. At step 902, thememory registers are cleared, the counter i is set equal to the integerone, and the operator chooses a maximum value N for this counter. Thisdetermines the maximum number of sequential pitch readings P_(i) thatwill be stored in a First-In-First-Out (FIFO) memory for averaging atstep 904. This will be an average of averages. The value of N may bedirectly entered utilizing the existing keys or keypad (not shown) onthe tracking receiver system 20. Its value may be adjusted as necessaryto reflect current soil conditions and/or drilling parameters. Theinitial setting may be determined by experimentation or estimated as inthe following example:

-   -   Typical Drilling Rate (for critical on-grade bores)≦3 ft/min    -   Threshold Increment (TI) for Pitch Updates=0.5 foot    -   (Maximum straight drilling interval over which the pitch        readings are to be averaged.)    -   Thus≦6 Pitch Updates/minute is sufficient    -   Present Interval between Pitch Updates=1 second=60/min        In this case, at least N=10 pitch readings P_(i) would be        sequentially collected at step 906 and stored in memory. The        average pitch P_(A) is recalculated every time a new pitch        reading P_(i) is stored. Once every register P_(Ri) of the        memory has been filled with readings, the average pitch P_(A)        relates to the last 0.5-foot drilled. That is, there will be        some “settling” of the readings P_(A) until the counter i=N. In        the event i>N (Step 908) the register values are shifted at Step        912 by decreasing the counter i by one (1), now equal to N,        which allows the newest pitch reading P_(i) to be stored as        P_(RN). After reduction of the counter (Step 912), the current        pitch reading Pi is placed in register P_(Ri) at Step 914.

Next, the average pitch P_(A) is calculated at Step 904. The processorthen decides at Step 916 which pitch value to display based upon fastrotation of the downhole tool 10. If the downhole tool 10 is not beingrotated fast the pitch P_(Ri) is displayed at Step 918. In the event thedownhole tool 10 is rotating fast the average pitch is displayed at Step920. At Step 922 the counter i is incremented and pitch is again read at906.

The filled memory is kept current on a FIFO basis by the fact that i=Nis increased to i=N+1. The register values (stored sequential pitchvalues) P_(Ri) for i=1 to N are shifted downward by one increment,causing the oldest one to be deleted. The displayed pitch (P_(i) orP_(A)) has been tailored to meet the differing needs of the two primarydrilling modes (i.e., corrective steering or straight drilling). Step916 automatically causes the displayed pitch be the one best suited forthe present mode.

The '484 Bieberdorf patent says that the tracking receiver decodes thepitch angle information transmitted by the beacon 26 and could utilizethat information to automatically control the directional downhole tool10 to correct or maintain a given pitch angle. Control logic and asuitable machine control system are disclosed in U.S. Patent ApplicationPublication No. 2004/0028476, the contents of which are incorporatedherein by reference. The disclosed machine control system canautomatically control operation of the drill unit 28 while guiding thedownhole tool 10 along a selected borepath segment 24. This isaccomplished via tracking and guidance control systems that obtain andutilize data indicative of the position and orientation of the downholetool 10 along with other data from the operation of the drill unit 28.For instance once at the target depth d₀ of the on-grade bore segment24, the guidance control circuitry advances the downhole tool 10 in astraight line. The tracking circuitry monitors the location andorientation of the downhole tool 10 and boring tool, communicating theinformation to the main control circuit. The information received fromthe tracking circuitry is documented to record the path of the boreholeas it is being bored. The location and orientation of the downhole tool10 can then be compared to the desired borepath 24. When the downholetool 10 veers from the intended borepath 24 (or when a new segment ofthe borepath calls for a change in direction), the guidance controlcircuitry will operate to change the direction of the downhole tool 10,guiding the downhole tool 10 along or back to the intended borepath.

Reactionary movement of the drill unit 28 is sensed and compensated for(e.g., when making drill string in-ground length calculations). Positionand movement of the carriage 142 is also sensed. Such sensing devicesand method are disclosed in U.S. Pat. No. 6,550,547 by Payne, et al.,the contents of which are incorporated herein by reference. Forwardmovement of the carriage 142 thrusts the drill string 12 through theearth 32. The operation of the carriage thrust motor (not shown) can becorrelated to the movement of the carriage 142 using a speed pickupsensor to count magnetic pulses from the revolving motor output shaft.An additional sensor or switch is used to indicate when the carriage 142has passed a “home” position. The magnetic pulses counted from the motorcan then be used to determine how far the carriage 142 has traveled fromthe home position and what direction it is currently traveling. It willbe appreciated that other means of determining carriage position andtravel exist including string potentiometers and magnetic pickupssensing rack teeth or fixed magnetic strips. Whenever drilling is inprogress, forward motion of the carriage 142, corrected by anyreactionary movement of the drill unit 28, can be utilized toapproximate the forward travel of the downhole tool 10. Alternatively,when a tracking receiver system of the type disclosed in U.S. PatentApplication No. 60/680,780 is utilized, sequential coordinatemeasurements of changing beacon position may be directly converted intodownhole tool 10 forward travel.

The automated drilling methods described above can be adapted toclose-tolerance on-grade applications by way of some enhancements to theflow diagram of FIG. 10. One approach to automate its averaging processis shown in FIG. 11. One skilled in the art could readily incorporatethe principles disclosed in this flow diagram into the disclosure ofU.S. Patent Application Publication No. 2004/0028476.

With reference now to FIG. 11, a method for automated drilling andbackreaming of an on-grade borehole is shown. At Step 1000 the system isinitialized and certain settings are entered. These settings mayinclude: (1) the distance to advance with rotation “DAWR”, (2) purgeprior pitch readings from memory, (3) threshold increment “TI” to 0.5 or1.0 feet, (4) sampling rate “SR” to acquire the desired pitch readingper foot of advance, (5) the desired grade “PS” in % grade, (6) thedesign tolerance “DT” of the desired grade, (7) the correction factor“CF” anticipated to be added to a pitch reading, and (8) the preferredor anticipated maximum distance D between tracking stations 36 (FIG. 1).Input of this information may be completed prior to beginning the bore,or before embarking on the on-grade segment. The program is active anytime automatic guidance is employed. At step 1004, the downhole tool 10is continuously monitored for forward movement and presence or absenceof rotation. Monitoring may be accomplished via techniques describedabove, or by utilizing tracking receivers known in the field. Steps 1006through 1018 are essentially equivalent to the flow chart of FIG. 10.

When the condition at step 1006 is not satisfied, the most recentlyreceived pitch reading P_(i) is displayed at step 1020. This provides amore responsive pitch indication whenever the downhole tool 10 advanceswithout rotation in the steering mode. If the condition at step 1006 issatisfied, the Distance Advanced With Rotation (DAWR) is measured withrespect to time and converted into essentially real time Drilling Rate(DR) at step 1008. A changing DR can be applied to adjust a normallytime-based sampling of pitch readings in the beacon to thedistance-based Sampling Rate (SR) utilized at Step 1010. For instance,this may be accomplished by varying the clock speed of the beacon'sAnalog to Digital Converter (ADC). The pitch readings P_(i) are storedin a FIFO memory of capacity determined by the multiplication of theThreshold Increment (TI) for Pitch Updates by the sampling rate SR. Onedifference from FIG. 10 is that the stored pitch readings P_(i) are notaveraged at step 1014 until step 1012 indicates all memory registershave been filled with readings. This removes display settling time. Theoperator is not left long without being given the first averaged pitchreading and subsequent updates follow quickly as described earlier.

Since the beacon pitch readings and the actual slope of the borepath maynot match exactly due to calibration inaccuracies or “drop” of the drillhead due to the action of gravity as the drill head rotates, at step1016 a Correction Factor (CF) is applied to the average pitch P_(A) toaccount for the difference between beacon pitch readings and actualslope of the borepath. The corrected average pitch P_(AC) is displayedat 1018 and compared to the specified grade or pitch PS at step 1022.The absolute value of their difference is calculated for comparison withthe more stringent of the ± tolerance band placed on PS (entered as apositive number in either case). In this instance an 80% factor has beenapplied so that the tolerance is not exceeded before straight drillingis stopped at Step 1024 for a confirming measurement of downhole tool 10position and orientation. So long as the condition in step 1022 is true,advancing with downhole tool 10 rotation (i.e., straight drilling) willcontinue until the next planned tracking station 36 is reached—asdetermined by the logic in step 1026.

The measurement at Step 1024 and need for corrective action at Step 1028is determined by one of the on-grade tracking systems of the presentinvention comparing the measurement to the pre-planned desired path 24.If no correction is needed, the straight drilling loop continues to theend of the on-grade segment of the bore is reached at Step 1030. If needof corrective steering is indicated at Step 1028, the correction 1032may be implemented in accordance with the teachings of U.S. PatentApplication Publication No. 2004/0028476.

When a correction in heading is required with typical HDD drill units28, the directional downhole tool 10 is rotated to the proper tool-faceheading (i.e., roll position). The drill string 12 is then thrustforward by advancing carriage 142 without rotation of the drill string.The directional downhole tool 10 deflects off its previous courseheading as it engages virgin soil beyond the point where rotationaladvance ceased. An opposite steering action may needed (especially forlateral direction corrections) before the downhole tool 10 fully returnsto the desired path, otherwise overshoot is likely to occur and theactual borepath will tend to zigzag around the desired path—apotentially unacceptable situation for close-tolerance installations ofproducts such as gravity flow drainage pipes. This circumstance can begreatly mitigated (or potentially avoided) by closely spacing thetracking stations 36 (FIG. 1)—which is more practical with the trackingreceiver systems of the present invention. Once deviation is detected,borepath planning algorithms such as described in the previouslyreferenced U.S. patent application Ser. No. 10/404,550 “Automatic PathGeneration and Correction System” may be adapted to provide steeringrecommendations that will direct the downhole tool 10 back onto thedesired path. This can be accomplished, for instance, by inputtingmultiple “critical points” that lay on the remaining (to be drilled)portion of the intended path 24.

Referring now to FIG. 12, an alternate embodiment of the presentinvention is illustrated. In the embodiment of FIG. 12, a laser-basedrobotic total station survey instrument 144 (such as the Topcon GPT-8200series or their GTS-820A) and a prism or target 146 to reflect adistance measurement signal from survey instrument 144 along line ofsight 148. This reflector 146 may be attached to a conventional tracker40 at a fixed height and orientation such that it is within the line ofsight 148 of the total station when the tracker is properly positionedfor obtaining a depth reading. One skilled in the art will appreciatethat a standard, manually sighted survey station such as the TopconGTS-230 Total Station could be utilized. However, robotic surveystations which can automatically track a target in the field areparticularly well suited for this application.

At the starting pit 34, the survey station 144 determines an initialhorizontal range R₀ and vertical declination D₀ (or elevation) of thetracker receiver system 20E relative to the instrument height of thesurvey station 144, designated in FIG. 12 as the Reference Elevation.The convention used in equations 1-4 assumes that values of Do which lieabove the Reference Elevation will be positive values and values of Dowhich fall below the Reference Elevation will be negative values. Sincethe position of the reflector 146 on the tracking receiver system 20E isfixed, the vertical distance from this target to the bottom surface ofthe tracking receiver system—designated as h_(F)—will remain constant.As above, the depth of the downhole tool 10 below the bottom of thetracking receiver system 20E is measured using traditional techniques.At the starting location this depth may be designated as d₀. Havingobtained the values of D₀, h_(F), and d₀, the initial elevation E_(H0)of the downhole tool 10 may be calculated by the equation:E _(H0) =D ₀−(h _(F) +d ₀)  (Eq. 1)

Its subsequent elevations E_(Hi) along the on-grade portion of the boreare obtained at appropriately spaced monitoring stations 36 _(i) wherethe total station 144 each time is used to acquire the horizontal rangeR_(i) and declination/elevation D_(i) of the tracking receiver system20E.E _(Hi) =D _(i)−(h _(F) +d _(i))  (Eq. 2)

As the bore progresses, it will be desired to maintain the downhole tool10 on the specified or target grade g_(T), which may be either up (+) ordown (−) depending on the desired flow direction of the installed pipe.For each range R_(i), along the borepath, there will be a targetelevation D_(Ti) for the downhole tool 10 relative to the ReferenceElevation that should be maintained to assure the bore is on-grade. Thistarget elevation D_(Ti) may be calculated at a given point along theborepath by the following equation:D _(Ti) =E _(H0) +g _(T)(R _(i) −R ₀)  (Eq. 3)Once the target elevation for the downhole tool 10 D_(Ti) and the actualelevation of the downhole tool 10 E_(Hi) are known at a given pointalong the bore, an error term representing any deviation in elevationfrom desired grade is calculated by the equation:Error=E _(Hi) −D _(Ti)  (Eq. 4)If the error term is negative, the actual elevation of the downhole tool10 is below the target elevation. Thus the downhole tool 10 will need tobe steered up to maintain grade (return to the specified grade). If theerror term is positive, the downhole tool 10 will need to be steereddown to maintain grade.

The use of the total station shown in FIG. 12 eliminates the need toshoot a detailed pre-bore topographic map of the area where the borewill occur. By relating all measurements to the Reference Elevation ofthe instrument 144, the system negates the effect of topographyvariations on the depth measurement d_(i) at the time measurements aretaken for each monitoring station 36 _(i).

Total station survey instruments have left-right angular (HorizontalAngle) measurement capability, which may be used to measure deviation ofthe downhole tool 10 from the desired boring azimuth (i.e., viameasuring the position of the tracking receiver 130 overhead). When theinstrument 144 is set up such that its “north” or zero azimuth directioncoincides with the bore alignment (assuming a straight left-rightalignment is specified), the coordinates outputted for each measurementstation 36 will directly indicate any left-right deviation. Where ahorizontal curvature is specified in the bore alignment, one skilled inthe art of surveying could readily derive equations similar to thoseabove to arrive at a lateral error term. In this case, an additionalsurvey point over the intended path would have to be taken wherever thedownhole tool 10 is found to have drifted laterally away. Although datareduction would involve more arithmetic, the total station 144 could beset up displaced from its preferred position in order to circumventobstacles that obscure line of sight to the reflector 146.

Many total stations are capable of wireless transmission 150 of therange and declination information they obtain for the target 146. Thisinformation may be relayed to the tracking receiver system 20E so thecalculations in the above equations can be performed there to provide animmediate display of the error term to the receiver operator 30.Alternatively, the range and declination information from the totalstation 144 may be received at the drill unit 28 along with downholetool 10 depth information d_(i) from the tracker 40, and the processingof the error term done by the receiving unit 46 (FIG. 1) at the drillunit. In either case, the information needed to correctly guide the boreon-grade is provided to crewmembers. Information such as targetelevation D_(Ti) of the downhole tool 10 (or a calculation of its targetdepth), left/right deviation, and actual elevation E_(Hi) of thedownhole tool (or its measured depth d_(i)) may be directly entered intoa bore mapping program such as the Subsite® Track Management System(TMS) for creation of an as-built map of the bore in profile view andoverhead view.

Alternatively, it is conceived the position of the surveyinstrumentation and the fixed target could be switched. Instrumentationproviding a precision inclinometer and range finder at the tracker couldbe used to generate a similar result to the scenario depicted in FIG.12. In this case a fixed target would be placed near the start of thebore, and the instrumentation at the tracker could shoot to the fixedtarget with every locate point to establish range and elevation of thenew point. However, there are practical difficulties in providing thestability of the instrumentation required to generate accurate readingsof range and elevation if the instrumentation were attached to themobile tracker. Thus, the set-up of FIG. 12 is the preferred embodiment.

With reference now to FIG. 13, an alternate on-grade tracking system 14Bmay be utilized to determine the FIG. 12 parameters of: horizontal rangeR_(i) to a target 152, its declination/elevation D_(i) and left/rightposition compared to the desired line of a bore designated to be placedat a specified grade. This system 14B utilizes a Real Time Kinematic(RTK) differential GPS survey system 154, 152 in place of the totalstation of FIG. 12. The surveying method involves initialization of abase receiver 154 at a known (or designated) coordinate position andelevation. The base receiver may utilize either the U.S. GlobalPositioning System (GPS) satellites 156 or the Russian GLONASSsatellites, or both, to acquire its position. The more accurate RTKsystems will take advantage of both the civilian (C/A) code and military(P) code from the GPS satellites to generate range measurements to thevisible satellites. Such systems are referred to as dual-frequency RTKsystems. The base receiver 154 calculates a correction factor for itsposition based on the received signals from multiple (four or more)satellites and relays 158 this correction factor to a moving “rover”receiver 152. Using the correction factor and GPS position informationgathered by the rover receiver, the data from the rover receiver may beprocessed to provide its position and elevation in effectively realtime. Current dual-frequency RTK systems are stated to have horizontalpositional accuracies of ±1 cm and vertical accuracies of ±2 cm.

The present embodiment 14B provides for the mounting of the roverreceiver 152 of a dual frequency RTK system at a fixed position on aconventional tracker 40 for a directional drilling system. In likemanner to the discussion of the use of a robotic total station 144 tomonitor the position of a tracking receiver system 20F, alternatively,the RTK system can provide relative elevation and distance data for thetracking receiver system 20F at any point along the borepath compared toinitial readings taken at the start of the on-grade segment.

Using the desired grade of the bore, the deviation of the downhole tool10 from its desired path may be calculated using the horizontal range,left/right positional data, change in elevation of the tracker, andmeasured depth of the downhole tool 10 using the tracking receiver 20F.Those skilled in the art will recognize that the calculation ofdeviation from grade using the RTK survey on-grade system 14B isfundamentally the same as that previously disclosed using the robotictotal station survey on-grade system 10A. The vertical distance from theroving receiver antenna 152 to the bottom surface of the trackingreceiver system—here again designated as h_(F)—will remain constant asbefore. The deviation from line will be calculated in much the same way.Utilizing the absolute position provided by the rover receiver 20E, thecalculation of whether the tracking receiver is to the left or right ofthe specified directional vector from the starting point isstraightforward.

The operator 30 may carry a portable computer linked to both the roverreceiver 152 and the tracking receiver system to perform thecalculations of deviation from grade and deviation from desired line.Alternatively, the computational electronics may be incorporateddirectly into a modified tracker or a purpose-built tracking receiversystem. A third alternative is to relay the data from the rover receiverand the tracker to the drilling unit 10 via a radio link 42 (FIG. 1) andhave the processing of the data done there.

While the ±2 cm vertical accuracy of current dual-frequency RTK systemsmay introduce some undesired undulations in the borepath, undulations ofthis magnitude may be removed during the backreaming and/or productplacement portion of the HDD process by utilizing bore straighteningreamers or steerable backreamer technology—the latter being disclosed inU.S. Patent Application Publication No. 2004/0188142 “DirectionalReaming System”, incorporated herein by its reference. It should benoted that steerable backreamers (and the variable product placementapparatus disclosed in this reference) typically have at least one pitchbeacon 26 (FIG. 1) on board. Thus one of the presently disclosedon-grade tracking systems may also be utilized during the backreamingportion of the HDD process to monitor and direct the path taken by thesteerable reamer and/or product pipe.

Most if not all, commercially available trackers are able to utilize thecharacteristic shape of the transmitted dipole magnetic field of thebeacon 26 to determine the direction in which the downhole tool 10 ispointing in a horizontal plane—i.e., its left-right heading. To do sowith some trackers, the device is positioned directly above the beacon26 via pinpointing techniques known in the industry. It is then rotatedabout its vertical axis until the signal strength impinging upon itsprimary receiving antenna (not shown) from the magnetic field isapproximately zero. One skilled in the art will appreciate that forclarity, only the basic tracker 40 is illustrated in FIGS. 14 and 15.One can appreciate that the apparatus and method now being describedwith respect to tracking may be employed with any of the previouslydescribed embodiments. As depicted in FIGS. 14 and 15, the sensorassembly is said to be in its “null” orientation at this point,perpendicular to the transmitting antenna of the beacon in thedirectional downhole tool 10 with its display 160 indicating zeroreceived signal strength. In other words, the tracking receiver is noworiented perpendicular to the present horizontal left-right heading 162of the downhole tool 10.

The tracker 40 illustrated in FIGS. 14 and 15 aids the operator tobetter visualize the actual horizontal heading or “line” 162 being takenby the downhole tool 10 in comparison to the intended path 24 (FIG. 1)that has been pre-marked on the ground surface 32 with a taut string164, marker flags, a paint stripe, or by other techniques. A laser“chalk line” 166 is mounted such that its “projected line” isperpendicular to the above-mentioned primary receiving antenna anddirected generally toward the ground 32. Thus, with the tracker 40oriented in its null position, the projected line gives a visualrepresentation (on the ground surface) of the direction the directionaldownhole tool 10 is pointing. In reality, the “projected line” consistsof outwardly fanning beams 168 of visible laser light forming apie-shaped plane, which projects into a line 170 when the beamsintersect an approximately perpendicular plane. The laser chalk line 166could be one of several commercially available handyman tools, so longas its projected light is adequately visible out of doors in brightsunlight.

The overhead view of FIG. 14 shows the downhole tool 10 in a phantomposition 172′ substantially to the right of the tracker 40, indicatingwhere it would likely be located if corrective action is not taken. Forimproved clarity, the present position of the downhole tool 10—directlyunder the tracking receiver—is not illustrated in FIG. 14. However, itis shown in the perspective side view of FIG. 15. For the same reason,the intended path 24 and its pre-marked representation 164 on the groundsurface 32 has been omitted from the latter figure. On the basis of theoverhead view illustration (FIG. 14), one can envision that these twolines would be angling out of the page toward the viewer. In otherwords, the downhole tool 10 has deviated leftward of the intendedborepath 24 when one is looking in the direction drilling isprogressing.

The projected “chalk line” 170 provides the tracker operator with arapid estimate of present left-right drilling direction 162 as comparedto the pre-marked desired direction 164. The disclosed invention aidsthe operator by providing him or her with a visible projection of thedirection of the downhole tool 10 along the surface of the ground toassist in decision-making of possible need for corrective steering inthe lateral (left-right) direction. Preferably the laser chalk linedevice 166 is mounted such that the vertical plane containing itsemitted beams 168 intersects the center of the tracking receiver'sprimary receiving antenna. To mount it otherwise would cause theprojected line 170 to not be directly above the downhole tool 10—insteadthe line would be offset, giving a laterally biased indication to theoperator.

Various commercial trackers utilize a variety of techniques to showdirection of the beacon. The above explanation of use with the nullingtechnique of a particular tracker is meant to be representative of howthe laser chalk line may be implemented and should not be construed asthe only way to marry the two technologies.

An alternate approach for displaying deviation of a downhole tool 10from the intended left-right line is the system illustrated in FIG. 16.To accomplish this particular task, the tracker 80 of U.S. PatentApplication No. 60/680,780 is centered on and aligned with a pre-markedrepresentation 174 of the intended borepath 24 (FIG. 1), thenperiodically moved forward along that ground surface marking 174 to staywithin range of a beacon 26 in the advancing downhole tool 10 (not shownin FIG. 16). Again, for the sake of clarity only the basic tracker 80 isillustrated in FIG. 16. One skilled in the art will appreciate that theapparatus and method of the previously described embodiments of thepresent invention may be employed with tracker of FIG. 16. The tracker80 has the capability to convert magnetic field measurements intoposition coordinates (x,y,z) and the left-right directional heading γ ofa beacon relative to itself without being directly above it. The beacon26 is shown in this overhead view as being a distance z forward of thecenter of the tracker 80 and at a distance y to the right. It is also atsome depth x below the ground surface. The present position andleft-right heading γ of the beacon 26 are measured and displayed 176relative to the present placement (position and orientation) of thetracker 80—now purposefully placed and aligned as mentioned above.Should the downhole tool 10 begin to veer off course laterally, theoperator can visually detect that occurrence and instigate a correctivesteering action. As is typical in the industry, pitch (a.k.a.,inclination or grade), roll angle and beacon operating parameters arealso available at the tracker. However, the present focus is directed toinformation relevant to the horizontal plane of the beacon 26—asdepicted in the overhead view of FIG. 16.

If it is assumed that the downhole tool 10 had previously drifted towardthe right, away from the desired line. FIG. 16 shows that an initialcorrective steering action has been implemented to turn the downholetool 10 back toward the desired line 174. It is likely that the initialsteering action should now or soon be ceased and the downhole tool 10 beadvanced with rotation to drill a straight interval. Or an oppositesteering action may be called for now (or after a short straightdrilling interval) to prevent overshoot of the intended path 174. Theoperator chooses one of these options for the next drilling interval andestimates how long to maintain that interval before position is againdetermined and compared to the intended path 174. Wrong choices mayresult in a more tortuous (zigzag) borepath, potentially unacceptablewhere close tolerance to the intended path is specified. The followingimprovement allows the operator to make a more informed choice betweenthese options before embarking on the next drilling interval.

As illustrated in FIG. 16, if maintained on its present heading (byinterjecting a straight drilling segment), the beacon 26 will intersecta vertical plane containing the planned borepath at a distance δ forwardof the antenna centroid of the tracking receiver. The tracker 80 of Ser.No. 60/680,780 already measures all the parameters necessary tocalculate this future intersection point P for the two bore paths (i.e.,the actual path intersecting the vertical plane of the intended path).The tracking receiver solves for y, z, and γ. From geometry:$\begin{matrix}{{\lambda = \frac{y}{\tan\quad\gamma}}{and}} & \left( {{Eq}.\quad 5} \right) \\{\delta = {\lambda + z}} & \left( {{Eq}.\quad 6} \right)\end{matrix}$The remaining drilling distance β to this intersection point is ofparticular interest:β=√{square root over (λ² +y ²)}  (Eq. 7)One can appreciate that the distance β can readily be corrected (andshould be) to account for the length L (not illustrated) from the centerof the beacon's transmitting antenna to the forward tip of the boringtool. This directly subtracts from the hypotenuse β of the intersectiontriangle, while the point of intersection P is unaffected by thisconsideration. Therefore the distance δ stays the same. The distances yand λ are reduced and the distance z increases. The primaryconsideration here is that the drilling distance to where the downholetool 10 intersects with the intended borepath 174 equals β−L. Thus asomewhat shorter drilling interval will be required to reach the desiredpath than illustrated in FIG. 16. This can be taking into account bydisplaying 176 numerically adjusted distances. To minimize tortuousity,the theoretical goal is that the downhole tool 10 be re-aligned with theintended path 174 at the same moment it intersects the intended path. Toapproximate this goal, the straight drilling interval has to be endedand the above-mentioned opposite steering action implemented. Thestraight drilling interval should be ended at an amount of undershoot(i.e., before arrival at P) determined by such factors as stiffness ofthe drill string 12 (FIG. 1) and soil parameters that likely vary fromone location to another. Such an adjustment can be estimated andmentally made by an experienced operator, or input by him/her to besubtracted from the “return to path” final straight drilling intervalestimate displayed 176 by the tracker 80 (and on the remote display 46of the drill unit 28 in FIG. 1). In close lateral tolerance drillingapplications, position and orientation of the beacon 26 may be checkedone or more times along this straight drilling segment to verify thatthe anticipated “corrective line” β is still being followed. It wouldnot be unusual to find that some of the angularity γ back toward theintended path 174 has been lost due to the resisting moment developed inthe drill string from the initial corrective steering action.

These above principles may be separately applied to assist the operatorin estimating the remaining drilling distance needed to bring avertically errant downhole tool 10 back onto specified grade and/ordepth. Here, a vertical plane containing the intended borepath comesinto play. If the left-right heading of the intended borepath follows acurved rather than a straight line, a differently oriented verticalplane might be involved for each new placement position of the tracker80. Parameters of concern are the pitch reading of the beacon 26 incomparison to the specified grade, and measured depth d_(i) of thebeacon in comparison to the target depth d₀ relative to the laserreference plane 22 of system 14.

Note that beacon depth is the coordinate “x” in the x-y-z coordinatesystem of the referenced Application. For the total station system 144or RTK GPS system 154 it is the depth error term relative to theReference Elevation that is of concern. (For a stand-alone tracker,depth x would be compared to pre-planned depth along the path). Thedepth differential (or depth error term) may be thought of as equivalentto “y” and the slope differential between the beacon's pitch reading andthe specified grade is (after conversion from % grade to angulardegrees) equivalent to “γ”. Equations 3 and 5 can now be used tocalculate the straight drilling interval β remaining to return to thespecified grade of the bore once the downhole tool 10 has been turnedback toward that line 174 by the initial corrective steering action(steer up or steer down, depending upon whether the deviation off coursewas downward or upward). Again, when correcting in the vertical plane,care must be taken to ensure that the beacon pitch reading does notbecome positive on a bore with a negative desired grade, and that thebeacon pitch reading does not become negative on a bore with a positivedesired grade. One skilled in the art could utilize the principlesdisclosed herein to derive equations for calculating thethree-dimensional “range back to target” from a point that is bothlaterally and vertically off course. In this instance, thethree-dimensional angular heading of the beacon with respect to thetracker 80 may be utilized. The heading is calculated from the magneticfield strength measured with three sets of orthogonal antennas 178. Thusthe operator can utilize the opposite of this heading for rollorientation or “clock face” of the final steering that will bring thedownhole tool 10 back onto the intended path 24 (FIG. 1).

The present invention is further directed to a method for creating anon-grade borehole. The method comprises establishing the reference line22 (FIG. 1) at a predetermined grade substantially aligned with thegrade of the desired subsurface path 24 (FIG. 1). The method includesdetermining the position of the downhole tool 10 with one of thepreviously described sensor assemblies and detecting the reference line22 at the receiving tracker system with a reference line receiver 50.Next, the distance between the reference line receiver 50 and the sensorassembly is measured and used in determining the position of thedownhole tool 10 relative to the reference line in conjunction withinformation regarding the position of the downhole tool.

In accordance with the present method, tracking receiver systemdisclosed herein may be used to determine the position of the downholetool in a horizontal plane. Once the horizontal location of the downholetool has been determined the sensor assembly may be used to detectmagnetic field signals emitted from downhole tool to determine the depthof the downhole tool below the sensor assembly. The reference linereceiver 50 may also be raised or lowered until it is impinged by thereference line. Upon impingement of the reference line receiver, thedistance between the sensor assembly and the reference line receiver ismeasured. The measured depth value and the measured distance between thereference line receiver and the sensor assembly are then used todetermine the distance between the reference line and the downhole tool.The operator may then use this distance to determine if the downholetool is on-grade with the desired subsurface path.

The present invention is directed to a method for creating an on-gradebore hole using an optical survey system as shown in FIG. 12. Inoperation, the downhole tool 10 is moved along the desired subsurfacepath and located at a point along the path using the tracking receiversystem 20E comprising a sensor assembly adapted to detect signalsemitted from the downhole tool 10. The optical survey system 144comprising a total station survey system measures the range andelevation of the tracking receiver system 20E relative to a startingabove-ground reference point 36 ₀ disposed along the desired subsurfacepath 24. A processor may be supported by the tracking receiver system20E and adapted to process the signals detected by the sensor assemblyand the range and elevation of the sensor assembly, transmitted from thetotal station 144, to determine the position of the boring tool 10.

Various modifications can be made in the design and operation of thepresent invention without departing from the spirit thereof. Thus, whilethe principal preferred construction and modes of operation of theinvention have been explained in what is now considered to represent itsbest embodiments, as herein illustrated and described, it should beunderstood that the invention may be practiced otherwise than asspecifically illustrated and described.

1. An on-grade horizontal directional drilling system comprising: adownhole tool operatively connected to a downhole end of a drill string,and steerable along a desired subsurface path; a reference linegenerator adapted to establish a reference line having a gradesubstantially the same as the grade of the desired subsurface path; atracking receiver system comprising: a sensor assembly adapted to detectsignals emitted from the downhole tool; a reference line receiveradapted to detect the reference line; a means for measuring a distancebetween the reference line receiver and the sensor assembly; and aprocessor adapted to process the signals detected by the sensor assemblyand the measured distance to determine the position of the downhole toolrelative to the reference line.
 2. The on-grade horizontal directionaldrilling system of claim 1 wherein the reference line generatorcomprises a laser transmitter.
 3. The on-grade horizontal directionaldrilling system of claim 2 wherein the laser transmitter comprises arotating beam laser level.
 4. The on-grade horizontal directionaldrilling system of claim 1 wherein the reference line receiver comprisesa laser receiver.
 5. The on-grade horizontal directional drilling systemof claim 4 wherein the laser receiver comprises a photo detector array.6. The on-grade horizontal directional drilling system of claim 1wherein the sensor assembly comprises at least two tri-axial antennaarrays.
 7. The on-grade horizontal directional drilling system of claim1 wherein the means for measuring the distance between the referenceline receiver and the sensor assembly comprises a string potentiometer.8. The on-grade horizontal directional drilling system of claim 1wherein the means for measuring the distance between the reference linereceiver and the sensor assembly comprises an ultrasonic distancemeasuring device.
 9. The on-grade horizontal directional drilling systemof claim 1 wherein the means for measuring the distance between thereference line receiver and the sensor assembly comprises a linearvariable differential transducer.
 10. The on-grade horizontaldirectional drilling system of claim 1 wherein the means for measuringthe distance between the reference line receiver and the sensor assemblycomprises a laser distance measuring device.
 11. The on-grade horizontaldirectional drilling system of claim 1 wherein the tracking receiversystem comprises an extendable frame member adapted to support thereference line receiver thereon.
 12. The on-grade horizontal directionaldrilling system of claim 11 wherein the extendable frame comprises afirst member adapted to support the sensor assembly and a second membersupported by the first member and adapted to support the reference linereceiver.
 13. The on-grade horizontal directional drilling system ofclaim 12 wherein the means for measuring the distance between thereference line receiver and the sensor assembly comprises a beaconsupported on the second member, wherein the sensor assembly is furtheradapted to detect a signal emitted from the beacon, and wherein theprocessor is adapted to process the signal emitted from the beacon todetermine the distance between the reference line receiver and thesensor assembly.
 14. The on-grade horizontal directional drilling systemof claim 12 wherein the means for measuring the distance between thereference line receiver and the sensor assembly comprises a magneticstrip disposed on either the first member or the second member and anencoder supported on the first member if the magnetic strip is disposedon the second member or on the second member if the magnetic strip isdisposed on the first member.
 15. The on-grade horizontal directionaldrilling system of claim 2 further comprising a longitudinal framemember adapted to support the sensor assembly and wherein the referenceline receiver comprises a plurality of laser sensors disposed along thelength of the longitudinal frame.
 16. The on-grade horizontaldirectional drilling system of claim 15 wherein the plurality of lasersensors each comprise a photo detector array.
 17. The on-gradehorizontal directional drilling system of claim 1 wherein the downholetool comprises a backreamer.
 18. The on-grade horizontal directionaldrilling system of claim 1 wherein the processor is further adapted todetermine the orientation of the boring tool relative to the referenceline.
 19. An on-grade tracking receiver system adapted to track theprogress of a boring tool, the boring tool being steerable along adesired subsurface path substantially parallel with a reference linehaving a grade, the tracking receiver comprising: a sensor assemblyadapted to detect signals emitted from the boring tool; a reference linereceiver adapted to detect the reference line; a means for measuring thedistance between the reference line receiver and the sensor assembly;and a processor adapted to process the signals detected by the sensorassembly and the measured distance between the reference line receiverand the sensor assembly to determine the position of the boring toolrelative to the reference line.
 20. The on-grade tracking receiversystem of claim 19 wherein the reference line receiver comprises a laserreceiver.
 21. The on-grade tracking receiver system of claim 19 whereinthe reference line receiver comprises a photo detector array.
 22. Theon-grade tracking receiver system of claim 19 wherein the sensorassembly comprises at least two tri-axial antenna arrays.
 23. Theon-grade tracking receiver system of claim 19 wherein the means formeasuring the distance between the reference line receiver and thesensor assembly comprises a string potentiometer.
 24. The on-gradetracking receiver system of claim 19 wherein the means for measuring thedistance between the reference line receiver and the sensor assemblycomprises an ultrasonic distance measuring device.
 25. The on-gradetracking receiver system of claim 19 wherein the means for measuring thedistance between the reference line receiver and the sensor assemblycomprises a linear variable differential transducer.
 26. The on-gradetracking receiver system of claim 19 wherein the means for measuring thedistance between the reference line receiver and the sensor assemblycomprises a laser distance measuring device.
 27. The on-grade trackingreceiver system of claim 19 further comprising an extendable framemember adapted to operatively support the reference line receiver. 28.The on-grade tracking receiver system of claim 27 wherein the extendableframe comprises: a first member adapted to support the sensor assembly;and a second member operatively connected to the first member formovement relative to the first member, wherein the second member isadapted to support the reference line receiver.
 29. The on-gradetracking receiver system of claim 28 wherein the means for measuring thedistance between the reference line receiver and the sensor assemblycomprises a beacon supported on the second member, wherein the sensorassembly is further adapted to detect a signal emitted from the beacon,and wherein the processor is adapted to process the signal emitted fromthe beacon to determine the distance between the reference line receiverand the sensor assembly.
 30. The on-grade tracking receiver system ofclaim 28 wherein the means for measuring the distance between thereference line receiver and the sensor assembly comprises a magneticstrip disposed on either the first member or the second member and anencoder supported on the first member if the magnetic strip is disposedon the second member or on the second member if the magnetic strip isdisposed on the first member.
 31. The on-grade tracking receiver systemof claim 21 further comprising a longitudinal frame member adapted tosupport the sensor assembly and wherein the reference line receivercomprises a plurality of laser sensors disposed along the length of thelongitudinal frame.
 32. The on-grade tracking receiver system of claim31 wherein the plurality of laser sensors each comprise a photo detectorarray.
 33. The on-grade tracking receiver system of claim 21 wherein thereference line receiver further comprises a visual indicator adapted toactivate when the reference line is centered on the photo detectorarray.
 34. The on-grade tracking receiver system of claim 19 wherein theprocessor is further adapted to determine the orientation of the boringtool relative to the reference line.
 35. The on-grade tracking receiversystem of claim 19 wherein the means for measuring the distance betweenthe reference line receiver and the sensor assembly comprises anelectromagnetic signal device adapted to transmit an electromagneticsignal to the sensor assembly.
 36. A method for creating an on-gradeborehole, the method comprising: establishing a reference line at apredetermined grade substantially the same as a grade of a desiredsubsurface path; determining a position of a boring toot with areceiving tracker system wherein the receiving tracker system comprisesa sensor assembly; detecting the reference line at the receiving trackersystem; measuring a distance between the reference line and the sensorassembly; and determining the position of the boring tool relative tothe reference line based on the determined position of the boring tooland the distance between the reference line and the sensor assembly. 37.The method of claim 36 wherein establishing a reference line comprisesgenerating a laser beam.
 38. The method of claim 37 further comprisingrotating the laser beam to define a plane.
 39. The method of claim 36wherein the boring tool comprises a beacon and wherein determining theposition of the boring tool with the receiving tracker system comprisesdetecting a signal emitted from the beacon at the receiving trackersystem.
 40. The method of claim 39 wherein the sensor assembly comprisesat least two tri-axial antennas adapted to detect the beacon signal,wherein determining the position of the boring tool comprises laterallyoffsetting the at least two tri-axial antennas from the boring tool. 41.The method of claim 36 wherein detecting the reference line at thereceiving tracker system comprises impinging a reference line receiverwith the reference line.
 42. The method of claim 41 wherein impingingthe reference line receiver further comprises raising or lowering thereference line receiver after determining the position of the boringtool.
 43. The method of claim 41 wherein impinging the reference linereceiver further comprises raising or lowering the reference linereceiver after determining a position of the boring tool within ahorizontal plane.
 44. The method of claim 36 wherein the position of theboring tool is determined with the receiving tracker system laterallyoffset from the boring tool.
 45. The method of claim 36 wherein theboring tool comprises a backreamer, the method further comprisingenlarging the on-grade borehole by retracting the backreamer.
 46. Themethod of claim 41 wherein measuring the distance between the referenceline and the sensor assembly comprises transmitting a laser beam from alocation proximate the reference line receiver to a target supportedproximate the receiving tracker system.
 47. The method of claim 41wherein measuring the distance between the reference line and the sensorassembly comprises transmitting an ultrasonic signal from a locationproximate the reference line receiver to a location proximate thereceiving tracker system.
 48. The method of claim 36 further comprisingwirelessly transmitting the measured distance between the reference lineand the tracker system to the receiving tracker system.
 49. The methodof claim 36 further comprising determining the position of the boringtool relative to the desired subsurface path.
 50. The method of claim 36further comprising determining a point of intersection between an actualpath of the boring tool and the desired subsurface path based on theposition of the boring tool determined by the receiving tracker system.51. The method of claim 36 further comprising calculating the distancebetween the position of the boring tool and the desired subsurface path.52. The method of claim 36 further comprising advancing the boring toolalong the desire subsurface path using the determined position of theboring tool relative to the reference line.
 53. The method of claim 36further comprising comparing a position of the boring tool relative tothe reference line at a point along the desired subsurface path to aninitial position obtained disposed near the start of the desiredsubsurface path to determine the need for steering corrections.
 54. Amethod for determining a position of a downhole tool relative to areference line, the reference line having a selected grade correspondingwith a grade of a desired bore path, the method comprising: locating thedownhole tool with a sensor assembly; measuring a distance between thedownhole tool and the sensor assembly; locating the reference line andmeasuring a distance between the reference line and the sensor assembly;and determining a distance between the reference line and the downholetool based on the measured distance between the downhole tool and thesensor assembly and the measured distance between the reference line andthe sensor assembly.
 55. The method of claim 54 wherein the downholetool comprises a beacon and wherein locating the downhole tool with thesensor assembly comprises detecting a signal emitted from the beacon.56. The method of claim 55 wherein the sensor assembly comprises atleast two tri-axial antennas adapted to detect the beacon signal andwherein determining the position of the downhole tool compriseslaterally offsetting the at least two tri-axial antennas from the boringtool.
 57. The method of claim 54 wherein locating the reference linecomprises impinging a reference line receiver with the reference line.58. The method of claim 57 wherein impinging the reference line receiverfurther comprises raising or lowering the reference line receiver afterlocating of the downhole tool.
 59. The method of claim 57 whereinimpinging the reference line receiver further comprises raising orlowering the reference line receiver after determining a position of thedownhole tool within a horizontal plane.
 60. The method of claim 57wherein the reference line receiver comprises a photo detector array andwherein impinging the reference line receiver comprises centering thereference line on the photo array.
 61. The method of claim 57 whereinmeasuring the distance between the reference line and the sensorassembly comprises transmitting a laser beam from the reference linereceiver to a target supported on the receiving tracker system.
 62. Themethod of claim 57 wherein measuring the distance between the referenceline and the sensor assembly comprises transmitting an ultrasonic signalfrom the reference line receiver to the receiving tracker system. 63.The method of claim 54 wherein the position of the downhole tool isdetermined with the sensor assembly laterally offset from the downholetool.
 64. The method of claim 54 further comprising determining theposition of the downhole tool relative to the desired subsurface path.65. The method of claim 54 further comprising calculating the distancebetween the downhole tool and the desired subsurface path.
 66. Themethod of claim 54 wherein the downhole tool comprises a backreamerassembly and wherein the method further comprises moving the backreamerassembly along the desire subsurface path using the determined positionof the backreamer assembly relative to the reference line.
 67. Anon-grade horizontal directional drilling system comprising: a downholetool operatively connected to a downhole end of a drill string, whereinthe downhole tool is steerable along a desired subsurface path; atracking receiver system comprising a sensor assembly adapted to detectsignals emitted from the downhole tool; an optical survey system adaptedto measure the range and elevation of the tracking receiver systemrelative to a starting above-ground reference point disposed along thedesired subsurface path; a processor adapted to process the signalsdetected by the sensor assembly and the range and elevation of thesensor assembly to determine the position of the boring tool.
 68. Theon-grade horizontal directional drilling system of claim 67 wherein theoptical survey system comprises a total station survey system.
 69. Theon-grade horizontal directional drilling system of claim 67 wherein theoptical survey system is adapted to automatically track the position ofthe tracking receiver system.
 70. The on-grade horizontal directionaldrilling system of claim 68 wherein the tracking receiver systemcomprises an optical reflector adapted to reflect an optical signalgenerated by the total station survey system.
 71. The on-gradehorizontal directional drilling system of claim 67 further comprising aprocessing means to compare the range and elevation of the trackingreceiver system at any point along a borepath to a target range andelevation based on a grade substantially equivalent to the desired gradeof the bore path.
 72. The on-grade horizontal directional drillingsystem of claim 67 further comprising a processing means to calculatethe lateral displacement of the tracking receiver system from thedesired subsurface path.